Nuclear power plants have traditionally been designed for achieving safe and reliable performance through the monitoring and analysis of various key operational parameters. Data derived from such monitoring components may be used to initiate emergency procedures, such as high and low pressure water injection into the reactor core in the event of an unexpected plant transient, as well as providing for operational control of feedwater flows, recirculating flows, containment levels, and like system operating processes. As such, there is a need for sensing components that accurately convey temperature, level, pressure, and flow rates of plant processes so that operating conditions may be accurately monitored. Where nuclear safety related components are involved, sensing component redundancy is generally employed for back-up and calibration purposes. Safety related system components or instrumentation used in nuclear related facilities must be certified to meet rigorous criteria of nuclear regulatory agencies such as the Nuclear Regulatory Commission (NRC) to assure long term operational reliability under extreme conditions. The NRC typically requires an IEEE electrical classification of 1E for these nuclear safety related components.
One such system monitoring component is a pressure transducer of a capacitive type. Such devices exhibit a change in capacitance relative to the pressure exerted upon the device's pressure sensitive surface which may then be electronically converted into a d.c. signal. In addition to providing data on operating pressures, process flow rates and liquid levels are often derived from measured pressure data, as well. Typical of the capacitive pressure transducers rated by the NRC and employed in nuclear safety related applications is the pressure transmitter. However, such transmitters, containing fluid-filled, pressure sensing, capacitive type pressure transducers have exhibited a tendency to lose internal dielectric fluid over a period of time through cracks and leaking seals. Since the capacitive output of such a pressure transducer is affected by the volume of dielectric fluid separating capacitive plates within the transducer, the corresponding d.c. output of such a leaking device will drift over time in response to a given pressure condition. Leakage can be tolerated to a degree as long as the functional integrity of the transducer can be assured. Without a method for accurately predicting when a given transducer is likely to reach a point of catastrophic failure, transducers may be prematurely removed as a result of an overabundance of caution--a "better safe than sorry" preventative maintenance program. In order to accurately monitor various fluid pressures throughout a nuclear power plant the fluid filled, capacitive type pressure transducers are periodically calibrated in order to detect, determine and accommodate for the effect of such leakage upon the integrity of the measured output of such devices.
Capacitive type pressure transducers are typically used to monitor wide ranging pressures (0-1200 p.s.i.g.) for a wide variety of plant processes. Transducers are generally rack mounted on local instrument panels or permanently installed directly into process lines within the plant. In the latter case, the transducers may be considered permanently installed and would not be removed for examination or testing because of the difficulty and expense of removing the often remotely-mounted devices. Access to permanently installed instrumentation may be difficult if not impracticable. In a nuclear power plant application, the problem of access to the devices is compounded by the presence of contamination since transducers may be systematically exposed to low levels of radiation or designed to function within a radiation field. Therefore, a method of remote calibration is preferred.
Over the recent past, various methods of calibration of such capacitive type transducers have been undertaken. One such method is to compare outputs from similar, redundant capacitive type transducers employed under similar operating conditions. Outputs of such redundant devices are then compared and cross-correlated to detect and evaluate the effect of any leakage on the output of the monitored transducer. Such cross-correlation of calibration data between redundant sensors may indicate some leaks, but only if the functional integrity of at least one of the transducers is assured. Further, unavoidable environmental process noise signals will often obscure effects of small leaks in the output signal. Therefore, calibration alone, in addition to being time consuming, is often ill-suited for the detection of small leaks.
Another method of calibration of such pressure sensors includes comparison of the outputs from non-redundant sensors, given a known source pressure. However, such data is of limited value since comparison against a standard output is often clouded by the presence of process noise fluctuations which produce a noise signal superimposed on a relatively constant value pressure indication. Even if transducers were available for removal for bench calibration using noise free sources of known pressures, such calibration suffers from the additional disadvantages of being labor intensive, obtrusive, potentially hazardous and requiring substantial coordination between operations and maintenance departments.
It is critical to assure that a capacitive type sensing device employed in a nuclear safety related application is functioning properly, even if its suffers from slow dielectric leakage. This assures that the device will accurately respond to fast and unexpected plant transients to which a nuclear power facility may be subjected. Therefore, a means of directly and accurately assessing the functional integrity of these pressure devices is desirable. Such a means would be most effective if it were non-intrusive, amenable to remote observation, continuous during normal operation, and capable of detecting and tracking small leakages even in the presence of process noise signals.